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US F-35 fighters on a training exercise in South Korea, 12 countries are due to operate the aircraft, including South Korea

Brutal civil wars in Syria and Yemen, coupled with the return of great power rivalries between the US, Russia and China, have brought the world's arms trade into sharp focus.

And unsurprisingly it is a thriving global industry, with the total international trade in arms now worth about $100bn (£74bn) per year, Pieter Wezeman, senior researcher at the Stockholm International Peace Research Institute (Sipri), tells the BBC.

And it is the United States that is extending its lead as the globe's number one arms exporter, adds Sipri.

It estimates that the US now accounts for 34% of all global arms sales, up from 30% five years ago, and are now at their highest level since the late 1990s.

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Saudi Arabia is now the world's top importer of US arms

"The US has been open to supplying arms to a large variety of recipients, and there are a large number of countries ready to acquire weapons from the US," says Mr Wezeman.

The US's arms exports are 58% higher than those of Russia, the world's second-largest exporter. And while US arms exports grew by 25% in 2013-17 compared with 2008-12, Russia's exports fell by 7.1% over the same period.

It is Middle East states that have been among the US's biggest customers - Saudi Arabia tops the list - with the region as a whole accounting for almost half of US arms exports during 2013-17.

Yemen's civil war

This comes as arms imports to the region have doubled over the past 10 years, driven by widespread conflicts across the area - most notably the civil wars in Syria and in Yemen, which the UN has called the world's worst man-made humanitarian disaster.

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About 75% of Yemen's population are in need of humanitarian assistance

Since Yemen's civil war started in 2015, Saudi Arabia and eight other Arab states have carried out an air campaign in support of forces loyal to President Abdrabbuh Mansour Hadi.

These are fighting Houthi rebels said to be backed militarily by Iran.

The bitter conflict in Yemen has brought the ethical issues of international arms sales into sharp relief in many western countries, which have seen Saudi Arabia and its allies use their advanced weapons systems in the country.

"Saudi Arabia, Egypt and the United Arab Emirates were major arms importers anyway," says Sipri's Pieter Wezeman. "The major difference is that now they are using these weapons - in Yemen."

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Saudi Arabia and eight other Arab states are carrying out air strikes to restore President Hadi's government

The UN says that Saudi-led coalition airstrikes continue to be the leading cause of child casualties as well as overall civilian casualties.

Meanwhile, rebel forces have fired artillery indiscriminately into cities such as Taizz and Aden, killing civilians, and also fired rockets into southern Saudi Arabia.

"There should be no doubt that the PLA [People's Liberation Army] today is no longer far behind the West when it comes to certain areas of defence technology," she says. "The West's superiority in the air is under growing threat.

"China may not yet be able to produce high-performance military jet engines, but with the rate they are innovating they are not light-years away from being able to do it."

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China's first aircraft carrier, the Liaoning: China is said to be planning to have four carrier battle groups

China's increased military spending comes as it is moving from being a land-based military to becoming a naval-based power - and has poured huge sums into its growing navy.

Since 2000 it has built more warships than Japan, South Korea and India combined - the total tonnage of new warships and auxiliaries launched in the last four years is greater than that of the French navy. Other countries across, such as Japan and India have responded by spending more on naval power.

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Asian military spending: Japan's latest helicopter carrier Izumo, though officially a "destroyer", is its biggest warship since World War Two

"China has grown at a staggering rate, economically, and is seeking to transform that into a military power that is consistent with a regional hegemonic position," says Veerle Nouwens, research analyst at Royal United Services Institute (Rusi).

Part of this strategy includes China's efforts to export its arms. It sold weapons to 48 countries during 2013-17, with Pakistan being its top customer, and it is making inroads into some of Russia's traditional export markets.

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India is also spending more; its defence imports rose 24% between 2008-12 and 2013-17 and is building six of these French-Spanish designed submarines

"They are both selling to similar customers - countries that the west won't sell arms to - like Iran, Venezuela, Sudan and Zimbabwe," says Dr Lucie Beraud-Sudreau of the IISS.

African conflicts

In a world where arms sales are rising, the major exception to this seems to have been Africa. Between 2008-12 and 2013-17 arms imports by African countries fell by 22%.

Yet crucially, the figures here do not tell the whole story. Internationally, arms sales are measured by the total value of the contract - but this downplays the significance of small arms and light weapons to continuing conflicts in Africa, most notably South Sudan's civil war.

"We are not seeing significant reductions in the fighting in South Sudan, and this is clearly being fuelled by significant purchases of small arms and light weapons," says Amnesty International's Oliver Feeley-Sprague.

"For instance, three shiploads of machine guns, which would make a huge difference to armed groups on the ground, yet would not even show up in the statistics."

In 2014 the Arms Trade Treaty (ATT) came into force, with the aim of regulating the international trade in conventional weapons.